It is known that water may contain impurities and contaminants, either soluble or non-soluble, e.g. in suspension, that may be harmful to human health even if present at ultra-trace levels. This concern has already given rise to numerous methods and systems of trace level water analysis. The contaminants, which are also called “analytes”, in the context of trace contaminant analysis media, apparatus and procedures, e.g. heavy metals, certain organic compounds, organic microorganisms, may be present at levels of parts per billion (ppb) or trillion (ppt), or less.
In the U.S. Pat. No. 5,512,491, a trapping medium of a microporous absorbent material is described, which provides entrapping of colloidal and other suspended matter present in water flowing through the medium. After a sufficient amount of the suspended matter has been entrapped in the medium, ultra-trace analysis of the entrapped analytes is carried out either by optical methods (photometry, fluoroscopy, spectroscopy or other) or by extraction.
Optical methods are usually more efficient and accurate for the trace analysis purposes. They typically require a source of excitation light for illuminating a sample containing analytes, causing it to emit a secondary light signal, e.g. transmitted, reflected, fluorescent, luminescent, scattered light or other, indicative of the presence and amount of analytes in the sample, and a detector for receiving the secondary light signal and interpreting it as a measure of fluid contamination.
Typically, the intensity of the secondary light signal is very low, as explained e.g. in the U.S. Pat. No. 4,245,910 (Kallander), where a scattered secondary light has been measured, which also varies strongly in various directions. Typically, samples containing analytes are unoriented emitters, which emit secondary light in the full 4π steradian angle. In addition, the level of the secondary light may be as low as individual photon count.
It is thus advisable to collect as much as possible of the secondary light signal at the detector to obtain a reliable contamination reading.
Such means have been known in the art. In early days, it has been suggested to use an integrating sphere for an improved light collection. However, it presents two practical problems, first, the optimum emission and detection foci are coincident at the centre of the integrating sphere, meaning the two optical intensities could not be discriminated. Secondly, the optimum positions of sample and detector are likewise coincidental. Thus, the mechanical requirements of locating these components are mutually exclusive.
In practice, one of the two optical functions of emitting or detecting light can be removed to the outside, being replaced, e.g. by a beam entering or exiting through a small opening in the integrating sphere. However, this immediately means that the sphere is degraded to a monofunctional optical component, rather than serving as a complete optical system. A description of single and double integrating spheres is provided, e.g. in the article by John W. Pickering, Scott A. Prahl, Niek van Wieringen, Johan F. Beek, Henricus J. C. M. Sterenborg, and Martin J. C. van Gemert, “Double-integrating-sphere system for measuring the optical properties of tissue”, APPLIED OPTICS, Vol. 32, No. 4, 1 Feb. (1993).
Other examples of efficient collection of light are described in the above mentioned U.S. Pat. No. 4,245,910, and also U.S. Pat. No. 4,188,543 issued to Brunsting et al.; U.S. Pat. No. 4,808,825 to Miyatake et al.; U.S. Pat. Nos. 4,200,802 and 3,946,239 to Salzman et al.; U.S. Pat. No. 4,861,163 to Bach; and U.S. Pat. No. 4,577,603 to Oehler et al. These references describe various types of reflective shells of an ellipsoidal or semi-ellipsoidal shape, which have two foci spaced from each other, and where the sample is disposed at one focal point, while the detector is placed at the other focal point to collect the secondary light emitted by the sample and reflected by the shell.
Certain other prior art applications using elliptical geometry include shock wave experiments, which focus an emission from one focus onto another focus, thereby creating a compressed liquid jet, see Gustafsson G., “Experiments on Shock-wave Focusing in an Elliptical Cavity”, J. Appl. Phys. 61, 1 Jun. (1987), and elliptical flash lamp setups for pumping solid state lasers, where the two-dimensional ellipsoidal geometry is used to deliver as much of the excitation energy to the lasing media as possible, see e.g. various laser cavity products manufactured by Directed Light Inc. in San Jose, Calif., USA as described in detail at http://www.directedlight.com/components/cavities.html(© 2004). It is therefore necessary to provide effective entrapping of contaminants present in the fluid to be analyzed, effective illumination of the entrapped contaminants to generate the secondary light of sufficient intensity, and to provide effective collection of the secondary light on the detector to ensure reliable measurements of the fluid contamination level.
In spite of the certain progress being made in the field of fluid contamination analysis, the need still exists in the industry for developing an improved apparatus for analyzing contaminants suspended in water or other fluids, which would be compact, portable, multi-functional, and have sufficient sensitivity for measuring trace amounts of contaminants.